1 Technical Field of the Invention
The present invention relates generally to an image-forming apparatus which is designed to convert an image carried by an image carrier into a visual image and transfer it onto a print sheet and which may be employed in copying machines, facsimile machines, printers, or information processing systems using electrophotographic printing techniques, and more particularly to an improved color shift correcting structure of such an image-forming apparatus.
2 Background Art
Typical color image-forming apparatuses are designed to form a multi-colored image by overlaying yellow, magenta, cyan, and block images in registration on a print sheet. A positional shift between the different color images on the print sheet will, thus, result in a shift between colors. In order to eliminate such a color shift, Japanese Patent No. 2505206 teaches printing marks continuously on a transfer belt for correcting a shift between color images. This structure is designed to correct locations of images formed by exposure of photosensitive drums on which the marks are to be printed and not the speed of the transfer belt. The distances from a sensor detecting the marks to each photosensitive drum and an image-transferred location of the transfer belt are long. A change in speed of the transfer belt in a long cycle (e.g., 1 m or more) may thus be corrected, but it is difficult to correct one in a short cycle (e.g., several tens mm).
The marks are first formed on the photosensitive drums and then transferred to the transfer belt. It is, thus, difficult to determine whether a positional shift of the marks arises from a change in speed of the transfer belt or a change in speed of the photosensitive drum, thereby resulting in a difficulty in correcting the speed of the transfer belt using the marks.
Japanese Patent First Publication Nos. 8-211693, 6-144631, and 9-62047 teach a belt speed correction structure in which two sensor are arrayed in lengthwise direction of a transfer belt to measure the time required for one of marks printed on the transfer belt to pass the two sensors for calculating the speed of the transfer belt.
The structure as taught in Japanese Patent First Publication No. 8-211693 prints a plurality of magnetic marks along the periphery of the transfer belt. Therefore, if the belt is partly deformed due to, for example, residual stress of the belt created during manufacture, it will result in a change in pitch of the marks, which makes it difficult to measure the speed of the belt accurately.
The structures as taught in Japanese Patent First Publication Nos. 6-144631 and 9-62047 print a plurality of marks with toner. The surface of a transfer belt is usually cleaned by a cleaner blade. A problem is, thus, encountered in that the marks may erased by friction between themselves and a cleaner blade. An additional problem is also encountered in that a consumption of toner is increased.
Typical color image-forming apparatus also have a sheet conveyer belt for transporting a print sheet to which images are to be transferred from a transfer belt. The sheet conveyer belt is usually moved by a drive roller. The drive roller is increased in temperature by radiation of heat from a neighboring fixer, leading to thermal expansion, which will result in a change in diameter of the drive roller. This change will cause the speed of the sheet conveyer belt to be changed, resulting in a shift between color images.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide an image-forming apparatus designed to measure the speed of image carriers such as photosensitive drums, a transfer belt, and a sheet conveyer belt accurately and eliminate positional shifts between colored images for avoiding a shift between colors.
According to one aspect of the invention, there is provided an image-forming apparatus which comprises: (a) an image carrier carrying images printed on one of a surface of the image carrier and a surface of a printing medium held on the image carrier, the image carrier having at least the surface made of a dielectric material; (b) an electrostatic mark writing device writing electrostatic marks at predetermined intervals on the image carrier during movement of the image carrier; (c) a detecting device detecting the electrostatic marks written on the image carrier during the movement of the image carrier to provide signals indicative thereof; and (d) a correction device correcting a speed of the movement of the image carrier based on the signals outputted from the detecting device.
In the preferred mode of the invention, the electrostatic mark writing device includes a plurality of electrostatic mark writing units which are arrayed in a direction of the movement of the image carrier and which form the electrostatic marks on the image carrier, respectively.
Each of the electrostatic mark writing units writes a series of the electrostatic marks at regular intervals. The intervals in the electrostatic mark writing units are different from each other. The correcting device performs a correcting operation based on the degree to which the electrostatic marks written by the electrostatic mark writing units are overlaid.
The detecting device may alternatively include a plurality of detecting units which are arrayed in a direction of the movement of the image carrier and which detect the electrostatic images, respectively.
According to another aspect of the invention, there is provided an image-forming apparatus which comprises: (a) an image carrier carrying images printed on one of a surface of the image carrier and a surface of a printing medium held on the image carrier, the image carrier having at least the surface made of a dielectric material; (b) an electrostatic mark writing device writing electrostatic marks at predetermined intervals on the image carrier during movement of the image carrier; (c) a detecting device detecting the electrostatic marks written on the image carrier during the movement of the image carrier to provide signals indicative thereof; and (d) a correction device correcting a location where each of the images is printed based on the signals outputted from the detecting device.
In the preferred mode of the invention, the electrostatic mark writing device includes a plurality of electrostatic mark writing units which are arrayed in a direction of the movement of the image carrier and which print the electrostatic marks on the image carrier, respectively.
Each of the electrostatic mark writing units writes a series of the electrostatic marks at regular intervals. The intervals in the electrostatic mark writing units are different from each other. The correcting device performs a correcting operation based on the degree to which the electrostatic marks written by the electrostatic mark writing units are overlaid.
The detecting device may alternatively include a plurality of detecting units which are arrayed in a direction of the movement of the image carrier and which detect the electrostatic images, respectively.